The present invention relates to sensors and in particular to optical sensors. There are both electrical and optical sensors capable of sensing pressure, stress, and penetration of objects. However, present electrical penetration sensors detect penetration of the sensor by a projectile via electrical shorts in the sensor. The penetrating projectile creates the short between two separated conductive layers. This short can be sensed and used to identify a penetration. One of the drawbacks of electrical sensors is the problem of inadvertent shorts of the two separated conductive layers. The conductive layers must be insulated from each other and the other conductive parts of the sensor and from the structure on which the sensor is mounted. This adds to the design, installation, and the overall troubleshooting and maintenance costs of the sensor. Electrical sensors can also generate inadvertent sparks, which in some applications, where explosives might be nearby, is highly undesirable. Electrical sensors are also subject to electrical noise that is generated from electromagnetic interference. Many situations where the detection of penetration is desirable are located close to highly explosive events, which events have been know to generate electrical noise. Noisy electrical signals can be difficult to interpret and can cause erroneous indication of penetration events. Another drawback of using electrical sensors is that the passing projectile can short out the signal cable. This event can erroneously be interpreted as a penetration event at the sensor or, even worse, short out the power supply and cause erroneous readings on other sensors that are connected to the same system. Electrical sensors have also proven to be susceptible to chemicals, which limits their applications.
Current optical sensors are not subject to the shorting problem. For example, ITT Industries, Advanced Engineering & Sciences of Reston, Va. offers a Photonic Hit Indicator. That sensor includes a grid of optical fibers. A projectile that penetrates the sensor cuts some of the optical fibers. Detection of the loss of optical signal in the severed fiber is used to identify the location of the projectile's penetration. This sensor is, however, an active sensor. That is, it requires light to be applied to the optical fibers of the sensor. Severing of the optical fibers by the penetrating projectile prevents the applied light from reaching photodetectors. Detecting the absence of the applied light on the optical fiber of the grid provides an indication of where the projectile penetrated the sensor. One disadvantage of this type of sensor is that a fine and precise layout of many optical elements is needed to achieve a fine spatial resolution of the impact point. In addition, such active layouts of optical fibers are expensive to manufacture. Also, like electrical sensors, they require power to drive the light source or sources for the optical fibers or fibers.
The prior art sensors discussed above provide for penetration time and location. They do not directly provide additional details on the trajectory of the projectile or other penetration characteristics of the projectile. In addition, both methods discussed above degrade significantly as projectile damage accumulates in multiple projectile scenarios. In the case of the electrical detection panels, a penetrating fragment will often leave the panel shorted out. Once a panel is shorted, it cannot detect the penetration of subsequent projectiles. In the case of the Photonic hit indicator, once a projectile penetrates, it creates blind spots at other locations where the same optical fibers run; each optical element is only capable of registering the first projectile passing through.
There are other systems that employ high-speed imaging to measure projectile trajectories. These systems are expensive to purchase and operate and are limited in use to very specific applications.